Method for finishing the surface of a rolling contact component

ABSTRACT

A method for finishing the surface of a rolling contact component, such as a bearing roller element, a bearing ring or a traction drive rolling element, comprises the steps of: carrying out a cutting operation on the surface, subsequently calendering or burnishing the surface by means of a ball comprising a material harder than the bearing component material whereby the maximum calendering or burnishing pressure exerted on the surface is less than 7 Gpa.

[0001] The invention is related to the field of finishing the surface of a rolling contact component. Such component can form part of a rolling element bearing, e.g. a bearing ring or a ball-type or roller-type rolling element. Also components which form part of a rolling traction drive are encompassed, e.g. the power rollers and discs of an infinitively or continuously variable transmission.

[0002] In the finishing process a proportioning and strengthening of the surface in question is obtained by means of a ball calendering (burnishing) tool which plastically deforms the roughness peaks which are left behind after a hard turning or rough grinding operation. The tool comprises a holder at the tip of which a ball is usually rotatably supported by mans of a pressure fluid. Typically, the calendering ball has a diameter of 2 mm to 13 mm.

[0003] As a result of the calendering or burnishing operation, the roughness peaks are plastically deformed, leading to a surface which typically has smooth material areas of equal height. A surface texture like this, that is with areas of about the same height, separated by grooves, is not filly smooth and is therefore suitable for the envisaged applications mentioned before, e.g. rolling element, power rollers etc.

[0004] A method of this type for manufacturing a traction drive rolling element has been disclosed in Japanese patent application JP-A-9137854. According to this prior art method, a surface pressures of 7 GPa or higher is applied. As a result, compressive residual stresses are obtained beneath the surface thus treated which lead to an increase of the rolling fatigue resistance and which would prevent the surface layers from flaking off. The method in question is claimed to generate compressive stresses of about 1.5 GPa at a depth of 0.1-0.2 mm beneath the surface.

[0005] Under specific conditions however, it appears that, despite the improved resistance against flaking, a decrease in life was obtained. Thus, the method should be adapted in such a way that this negative effect does not occur.

[0006] The object of the present invention is therefore to provide a method for finishing a surface of a rolling contact component which combines excellent tribological propertied with a long contact fatigue life, good scuffing resistance and a good ability to build up a lubricant film.

[0007] This object is achieved by a method for finishing the-surface of a rolling contact component, such as a bearing rolling element, a bearing ring or a traction drive rolling element, comprising the steps of:

[0008] carrying out a cutting operation on the surface,

[0009] subsequently calendering or burnishing the surface by means of a ball comprising a material harder tan the rolling contact component material whereby the maximum calendering or burnishing pressure exerted on the surface is less than 7 GPa.

[0010] By controlling the calendering or burnishing pressure at values less than 7 GPa, first of all the desired pattern or texture of flattened areas or lands is obtained, which areas are separated from each other by grooves. The flattened areas have the same height, which results in a balanced load sharing ability. On the other hand, the limitation of the calendering pressure provides reduced residual surface and subsurface compressive stresses, which leads to an extended life.

[0011] Preferably, compressive stresses are generated at and/or beneath the surface, which compressive stresses do not exceed 1 GPa absolute.

[0012] Most preferably, the compressive stresses do not exceed 0.8 GPa absolute.

[0013] Since the surface of the component in question has been overrolled and thereby plastically deformed, it can be expected that the running in takes place already at the manufacturing stage. The bearing therefore does not require running in.

[0014] The process in question can be carried out in a quick and cost-effective manner by combining the (hard) cutting operation and the calendering operation in a single pass turning process.

[0015] The calendering ball may either be of a rolling type or of a sliding type.

[0016] Furthermore, specific surface portions may be exposed to the calendering or burnishing pressure for more than one time.

[0017] This can be achieved by selecting the advancement speed of the tool which carries the calendering ball in such a way in relation to the turning rotation speed of the object that each spot on the surface is more than one time, e.g. five times, contacted by said ball.

[0018] The invention will describe further with reference to the figures.

[0019]FIG. 1 shows a state of the art surface obtained by grinding.

[0020]FIG. 2 shows a surface obtained according to the method of the invention.

[0021] The surface shown in FIG. 1, e.g. of a bearing ring or power roller, has been obtained by a cutting operation and subsequent grinding.

[0022] The surface has a regular pattern of grooves 2 and ridges 1, providing ISO reference of R_(A) 0.2 μm.

[0023] The surface as shown in FIG. 2, obtained by a hard cutting operation and subsequent calendering or burnishing, comprises grooves 3 between areas 4. The toughness R_(A) in general is 0.27 whereas the roughness on the areas (which need not to be fully flat) is R_(A) 0.1 microns.

[0024] It is of importance that the areas 4 all have the same height, whereby a perfect load sharing ability is obtained. As a result, the fatigue life is almost equal to that of very smooth surfaces.

[0025] According to the invention, the surface and subsurface residual compresses stresses are less than 0.8 GPa. 

1. Method for finishing the surface of a rolling contact component, such as a bearing rolling element, a bearing ring or a traction drive rolling element, comprising the steps of: carrying out a cutting operation on the surface, subsequently calendering or burnishing the surface by means of a ball comprising a material harder than the rolling contact component material whereby the maximum calendering or burnishing pressure exerted on the surface is less than 7 GPa.
 2. Method according to claim 1, wherein compressive stresses are generated at and/or beneath the surface, which compressive stresses do not exceed 1 GPa absolute.
 3. Method according to claim 2, wherein the compressive stresses do not exceed 0.8 GPa absolute.
 4. Method according to claims 1, 2 or 3, wherein a hard cutting operation is carried out on the surface before the calendering or burnishing operation.
 5. Method according to any of claims 1-4, wherein the cutting operation and the calendering or burnishing operation are carried out in a single pass turning process.
 6. Method according to any of the preceding claims, wherein the calendering or burnishing operation is carried out by means of a ball comprising a material harder than the rolling contact component material.
 7. Method according to claim 6, wherein the ball is rolled over the surface.
 8. Method according to claim 6, wherein the ball is slid over the surface, a high traction fluid being provided for increasing the sliding force between said ball and surface.
 9. Method according to any of the preceding claims, wherein specific surface portions are exposed to the calendering or burnishing pressure each more than one time.
 10. Method according to claim 9, wherein the ball is rolled over the surface maximally five times. 